1. Field of the Invention
The present invention relates to an encoding apparatus, an encoding method, and an a computer-readable recording medium suitable for use at the time when conducting compression encoding on image data.
2. Description of the Related Art
With spread of image recording devices corresponding to digital Hi-Vision broadcasts and HD (High Definition) images, and attainment of higher resolutions of digital cameras, larger screens and higher resolutions are promoted in TV sets used in homes as well. As for an LSI (Large-Scale Integration) which conduct image processing in TV sets as well, therefore, demands for LSIs corresponding to HD images are increasing.
By the way, as for enhancer, frame rate conversion processing and so forth in the image processing, it is necessary to store image data of at least one frame in a memory allowing fast access, such as a DDR SDRAM (Double-Data-Rate Synchronous Dynamic Random Access Memory). Such a memory which stores image data is demanded to access fast, and consequently such a memory is expensive. Furthermore, in some cases, a plurality of large-capacity memories are needed to store a plurality of frames of image data of the above-described HD image. This causes an increase in product cost and the number of parts.
Therefore, a technique of conducting compression encoding on image data and storing resultant image data has been developed conventionally. As one of compression encoding systems of image data, the MPEG (Moving Pictures Experts Group) system is known. Since the algorithm is complicated, however, a large area is occupied when the circuit is formed by using an LSI. Therefore, the MPEG system is not suitable for application to signal processing LSIs specialized for high picture quality processing of images or inexpensive image processing LSIs. JP 7-15347 A, discloses a method of conducting compression encoding by predicting a pixel value of an object pixel in a one-dimension form by using a pixel value of a pixel next to the object pixel in a line direction (horizontal direction) and completed in encoding, prior to encoding of the object pixel.
Furthermore, a method of predicting a pixel value of an object pixel in a two-dimensional form and conducting compression encoding by using pixel values of a pixel on a position (horizontal position) on a line corresponding to the object pixel on a line immediately preceding a line including the object pixel in encoding order and neighboring pixels is also known. It becomes possible to improve the encoding efficiency by using the method of predicting a pixel value in a two-dimensional form as compared with a case where a pixel value is predicted in a one-dimensional form.
By the way, in recent years, the technique concerning the so-called 3D video in which a two-dimensional image is displayed stereoscopically has advanced and devices for supplying 3D videos have also spread. As one of transmission formats of 3D video, there is an LBL (Line-by-Line) system in which image signals for the left eye and the right eye are disposed and displayed on every other line alternately.
In the LBL system, it is considered that correlation between adjacent lines is small as compared with ordinary images. In a case where the above-described compression encoding system in which a pixel value is predicted in the two-dimensional form is applied to an image signal according to the LBL system, therefore, if a prediction direction is determined in a position other than the object pixel position, there is a possibility that the prediction direction might differ from a prediction direction in which a prediction error becomes minimum in the actual object pixel position. In this case, the prediction error in the longitudinal direction becomes large sometimes. This results in a problem that there is a possibility that the prediction error will propagate in a downward direction in some cases.